Advanced Materials series: Using metamaterials to make versatile sensors
What are metamaterials?
Metamaterials are made from the assembly of multiple composite elements such as metals and plastics. Metamaterials are purposefully designed to be able to control electromagnetic (EM) waves through blocking, absorbing, enhancing or bending waves. This unique ability arises not from the properties of the constituent materials but from the specific, geometrical arrangement of these subunits, at scales much smaller than the wavelength they wish to manipulate.
Metamaterials include several classes of EM composites such as negative index materials, photonic crystals, zero index materials, low index materials and chiral materials. Depending on which class they are in, metamaterials could be applied to the following; sensors, light emitting diodes, solar panels and high gain antennas.
Credit: Shin-Liang Chen, Ching-Shiang Chao & Yueh-Nan Chen
What industries use metamaterials?
Metamaterials are gradually being incorporated in to devices and applications, two prominent sectors include the security and communication industries.
The security sector is beginning to use Evolv Edge, which is a self-contained security screening system. It uses metamaterial antenna for millimetre wave (MMW) imaging. This imaging technique is one of a few that can penetrate through walls, dielectric (insulator) materials and clothing whilst using a non-ionising frequency, thus remaining harmless. It can detect metal and non-metal items and its application in artificial intelligence (AI) for facial recognition could see it being used as a screening tool for employees and visitors. To date, it is being used in Oakland International Airport as one of the screening requirements for its employees. The device can screen a person in less than five seconds without the need to pause or decrease pace and hence has increased operational efficiency significantly.
The communication industry is using flat-panel satellite antenna to better mobile connectivity. Kymeta has produced Land Mobile satellites that are affixed to the top of vehicles such as coaches. They can then be used to monitor routes whilst offering enhanced connectivity. As well as this, Echodyne has produced radars combining their Metamaterial electronically Scanning Array (MESA) technology and Acuity software. These radars have applications in security and autonomous vehicles.
How will metamaterials impact the rail industry?
One application of metamaterials is on the flat-panel satellite antennas on the roofs of trains which may improve on-board connectivity.
Another application currently in development is the use of metamaterials in LIDAR (the system used to sense surroundings for unmanned devices such as autonomous cars, drones and so on). LIDAR is used by the rail industry for obstacle detection and structure gauging. Metamaterials can be used to significantly enhance LIDAR and improve the effectivity and efficiency of these systems. They could also accelerate developments in autonomous vehicles, which will have potential knock-on effects for the rail.
What uncertainties remain?
There are some drawbacks to metamaterials which are preventing their implementation in everyday devices. These include, but are not limited to;
- Limited range
- Inability to alter shape
As stated above, a metamaterial is only able to manipulate EM waves if its subunits are smaller than the wavelength of the EM wave. This poses difficulties in producing metamaterials that can manipulate radiation with small wavelengths such as visible light (wavelength of between 380-750 nanometres). A metamaterial’s unique ability to manipulate EM waves is dependent on fixed geometrical pattern. Therefore, even minor changes to their shape could change their wavelength of operation or result in complete loss of function.
In addition, due to the constraints of length, it is not known if it is even possible to produce a metamaterial that is able to manipulate more than one type of wave on the EM spectrum. Thus, it may not be possible to produce multi-functional metamaterials.
The fabrication of metamaterials requires improved methods with better resolution and precision to turn theories into practical applications. Improvements in nanotechnology could help to overcome such problems in the future.
What is the current state of R&D?
Since 2010, when the Metamaterials Research Group was set up at the University of Birmingham with a £1.5 million grant. Since then, investments in metamaterials research has grown and in 2014, Imperial College London was awarded a grant of £2.5 million to research the different applications and properties of metamaterials such as, elasticity, acoustics, structural mechanics and diffusion. Other organisations are also investigating acoustic properties; Hamburg and Montreal aviation regions launched a research and development program in 2017 to create quieter aircraft cabins, partnering with universities, companies and research institutions in the two countries. Acoustic metamaterials are being investigated as a potential material for sound-absorbing insulation as part of this program.
Investment figures jumped again when, in 2017, Sustainable Development Technology Canada (SDTC) invested $5.4 million in Metamaterial Technologies Inc. (MTI) for the development and commercialisation of lightweight and high-efficiency silicon-based solar cell technology suitable for the transport industry. By 2025, it is estimated that metamaterial manufacturing will have a multiple billion-dollar market.
What should the rail industry do?
Potential applications in the rail industry include the Kymeta flat satellites to be used for improving on-board connectivity by delivering consistent high bandwidth. This could help to increase productivity in commuters as well as improve how passengers are kept informed during disruptions. In addition, the Evolv Edge security screening device could be deployed at either the entrance to stations or before ticket barriers to heighten security and increase public trust in the rail industry.
As well as funding these research projects which may have a direct impact on rail, the industry could undertake its own rail-centric research. This could include initiatives that specifically look at the implementation of metamaterials in rail-only applications, such as the potential to use metamaterials in the structure of rolling stock to reduce weight and increase energy efficiency.